Dc monitoring system for variable frequency drives

ABSTRACT

A DC monitoring system is configured to measure and analyze VFD (Variable Frequency Drive) operational characteristics. The VFD is configured to receive three-phase input power from a standardized source and to provide variable frequency three-phase output power to a three-phase motor. In some applications, the VFD and motor operate at a medium voltage. The VFD can include multiple inverter modules consisting of a DC section and switching section, also referred to as a multiple bus configuration. The DC monitoring system includes a measurement module coupled to each DC bus of the VFD, a data communication network, and a PQube monitoring device for transmitting data signals corresponding to voltage values of the VFD DC bus obtained in a medium voltage compartment to a low voltage compartment for processing and analysis. Processing of the data signals enables comparative and predictive analysis to determine early warning for possible capacitance failure in the VFD.

RELATED APPLICATIONS

This patent application claims priority under 35 U.S.C. 119(e) of theco-pending U.S. provisional patent applications, Application Ser. No.62/802,134, filed on Feb. 6, 2019, and entitled “DC Monitoring Systemfor Variable Frequency Drives”, which is hereby incorporated in itsentirety by reference.

FIELD OF THE INVENTION

The present invention is generally directed to the field of variablefrequency drives. More specifically, the present invention is directedto a DC (direct current) monitoring system for variable frequencydrives.

BACKGROUND OF THE INVENTION

Three-phase motors are commonly used in industrial applications. Apreferred method for controlling the speed of a three-phase motor is tovary the voltage and frequency of a three-phase AC voltage input to thethree-phase motor. A variable frequency drive (VFD) is a device thatprovides variable voltage and frequency to rotate a three-phase motor atany speed based on a control signal. The VFD can be controlled to adjustthe voltage and frequency of a supplied three-phase AC voltage accordingto speed requirements of the three-phase motor. Common applications forVFDs are in pumps, blowers, conveyors, and induction generators inindustries such as manufacturing, water treatment, desalination plants,paper mills, engine propulsion and HVAC.

A typical VFD operates by first changing an input three-phase AC voltageinto DC voltage, and then changing the DC voltage back to anotherthree-phase AC voltage at a desired voltage and frequency. It isunderstood that there are also single-phase VFDs that receivesingle-phase AC voltage as input and output another single-phase ACvoltage at a desired voltage and frequency. The following can also beapplied to single-phase VFDs FIG. 1 illustrates a functional blockdiagram of a conventional VFD. The VFD includes a diode converter 2(commonly a bridge rectifier), a DC bus 4, and inverter 6, and controllogic 8. The diode converter 2 converts an input three-phase AC voltageto a rectified three-phase DC voltage. A first phase of the three-phaseAC voltage is represented as L1, a second phase as L2, and a third phaseas L3. The DC bus 4 filters the rectified three-phase DC voltage byusing large smoothing capacitors, and outputs a filtered three-phase DCvoltage that is input to the inverter 6. The inverter 6 converts the DCvoltage back into another three-phase AC voltage at a specific frequencyas required by a three-phase motor 10. The control logic 8 controlsoperation of the inverter 6 to set the specific frequency of thethree-phase AC voltage output to the three-phase motor 10. The specificfrequency of the three-phase AC voltage received by the three-phasemotor 10 determines a speed (rotations per minute) of the three-phasemotor 10. If the three-phase motor 10 is to operate at a differentspeed, then the inverter 6 is controlled by the control logic 8 tooutput a three-phase AC voltage at a different frequency sufficient foroperating the three-phase motor 10 at the different speed. In thismanner, a VFD maintains a voltage/Hertz ratio, for example 480V/60 Hz=8.If the motor is to rotate at half speed both the voltage and frequencymust be halved, for example 240V/30 Hz=8. Failure to maintain the ratioresults in the motor overheating.

FIG. 2 illustrates an exemplary schematic diagram of an implementationof a low voltage version of the VFD of FIG. 1. The converter 2 includesa bridge rectifier circuit, such as a diode bridge shown in FIG. 2. TheDC bus 4 includes a link inductor (L) and smoothing capacitor (C)filter, represented by simplified LC filter shown in FIG. 2 (the linkinductor L is optional). The inverter 6 includes switching devices, suchas IGBTs (insulated gate bipolar transistors) shown in FIG. 2, that areeach controlled by driving signals provided by the control logic 8. Thecontrol logic 8 generates control signal pulses used to control theoutput of the switching devices in the inverter 6 in the proper sequencefor generating the three-phase AC voltage supplied to the three-phasemotor 10.

Unexpected failure of a VFD causes downtime of the connected motor andloss of revenue while a replacement VFD is found and installed. Findingan identical VFD may not be possible, leading to longer downtime whilerelated electrical and control systems are modified for thenon-identical VFD replacement. VFDs may fail due to DC bus failure(capacitor failure), input rectifier failure, output IGBT failure, oroverload. It is desirable to preemptively detect if a VFD is going tofail. VFD manufactures typically suggest replacing capacitors every 5-7years regardless of their performance, which is expensive and requiressignificant downtime of the VFD. This is an expensive overhaul that maynot be necessary.

SUMMARY OF THE INVENTION

Embodiments are directed to a DC monitoring system connected to a VFD.The DC monitoring system is configured to measure and analyze VFDoperational characteristics. The VFD is configured to receivethree-phase input power from a standardized source and to providevariable voltage and frequency three-phase output power to a three-phasemotor. In some embodiments, the VFD and motor operate at a mediumvoltage (voltages between 2001 VAC and 35,000 VAC). In some embodiments,the VFD includes multiple DC buses, also referred to as a multiple DCbus configuration. The DC monitoring system includes a measurementmodule coupled to each DC bus of the VFD, a data communication networkincluding a data concentrator, and a PQube monitoring device thatacquires, computes and stores data for transmitting signals of thecorresponding voltage values of the VFD DC bus obtained in a mediumvoltage compartment. The DC modules exist in medium voltage compartmentand the data concentrator and PQube monitoring device are located in alow voltage compartment for processing and analysis. In someembodiments, the data communication network between the DC buses and thedata concentrator is an optical network, such as a non-conductive fiber.The data communication network provides ground isolation and no directelectrical conduction path from the medium voltage compartment into thelow voltage compartment. Processing of the data signals enablescomparative and predictive analysis to determine early warning forpossible capacitance failure in the VFD.

In an aspect, a system to measure operational characteristics of avariable frequency drive is disclosed. The system includes a mediumvoltage component, a low voltage component, and a physical barrier. Themedium voltage compartment comprises a medium voltage variable frequencydrive configured to receive as input a medium voltage, wherein themedium voltage variable frequency drive comprises multiple DC buses,each DC bus having a low voltage; a plurality of measurement modules,one measurement module coupled to one DC bus, wherein each measurementmodule is configured to measure a voltage value of the DC bus andconvert the measured voltage value to a data signal; and a plurality ofoptical fibers, one optical fiber coupled to one measurement module, andeach optical fiber configured to transmit the data signal. The lowvoltage compartment comprises a PQube measurement device coupled to theplurality of optical fibers to receive the data signals, wherein thePQube measurement device is configured to process the received datasignals to determine operational characteristics of each DC bus. Thephysical barrier separates the medium voltage compartment and the lowvoltage compartment, wherein the physical barrier is configured to allowthe plurality of optical fibers to pass therethrough. In someembodiments, the medium voltage comprises a voltage value in the rangeof 2001 VAC to 35,000 VAC. In some embodiments, the low voltagecomprises a voltage value in the range of 0 to 2000 VAC. In someembodiments, each DC bus comprises a filtering capacitor, and themeasured voltage value of the DC bus corresponds to a voltage valueacross the filtering capacitor. In some embodiments, the measurementmodule is powered by the DC bus. In some embodiments, the low voltagecompartment further comprises a data concentrator module coupled betweenthe plurality of optical fibers and the PQube measurement device.

In another aspect, another system to measure operational characteristicsof a variable frequency drive is disclosed. The system includes a mediumvoltage component, a low voltage component, and a physical barrier. Themedium voltage compartment comprises a medium voltage variable frequencydrive configured to receive as input a medium voltage in the range of2001 VAC to 35,000 VAC, wherein the medium voltage variable frequencydrive comprises multiple DC buses, each DC bus having a low voltage inthe range of 0 VDC to 2000 VDC; a plurality of measurement modules, onemeasurement module coupled to one DC bus, wherein each measurementmodule is configured to measure a voltage value of the DC bus andconvert the measured voltage value to a data signal, further whereineach measurement module is powered by the one DC bus, and a plurality ofoptical fibers, one optical fiber coupled to one measurement module, andeach optical fiber configured to transmit the data signal. The lowvoltage compartment comprises a PQube measurement device coupled to theplurality of optical fibers to receive the data signals, wherein thePQube measurement device is configured to process the received datasignals to determine operational characteristics of each DC bus. Thephysical barrier separates the medium voltage compartment and the lowvoltage compartment, wherein the physical barrier is configured to allowthe plurality of optical fibers to pass therethrough to transmit thedata signals from a medium voltage compartment to the low voltagecompartment. In some embodiments, each DC bus comprises a filteringcapacitor, and the measured voltage value of each DC bus corresponds toa voltage value across the filtering capacitor. In some embodiments,each measurement module is powered by the corresponding one DC bus. Insome embodiments, the low voltage compartment further comprises a dataconcentrator module coupled between the plurality of optical fibers andthe PQube measurement device.

BRIEF DESCRIPTION OF THE DRAWINGS

Several example embodiments are described with reference to thedrawings, wherein like components are provided with like referencenumerals. The example embodiments are intended to illustrate, but not tolimit, the invention. The drawings include the following figures:

FIG. 1 illustrates a functional block diagram of a conventional VFD.

FIG. 2 illustrates an exemplary schematic diagram of an implementationof a low voltage version of the VFD of FIG. 1.

FIG. 3 illustrates a schematic block diagram of a VFD operating at lowvoltage according to some embodiments.

FIG. 4 illustrates a schematic block diagram of a medium voltage VFDhaving multiple DC buses according to some embodiments.

FIG. 5 illustrates a functional block diagram of the DC monitoringsystem configured for transmitting data from a medium voltagecompartment to a low voltage compartment according to some embodiments.

FIG. 6 illustrates exemplary low voltage three-phase AC voltagewaveforms across each secondary winding in the medium voltage VFD orinput waveforms to a low voltage VFD according to some embodiments.

FIG. 7 illustrates a rectified low voltage three-phase DC voltagewaveform corresponding to the low voltage three-phase AC voltagewaveforms of FIG. 6.

FIG. 8 illustrates a voltage ripple waveform corresponding to therectified low voltage three-phase DC voltage waveform of FIG. 7 withouta smoothing (filter) capacitor in the DC bus.

FIG. 9 illustrates an exemplary voltage ripple waveform where the DC bushas a new capacitor.

FIG. 10 illustrates an exemplary voltage waveform where the DC buscapacitor is aged.

FIG. 11 illustrates an exemplary voltage ripple waveform where the DCbus capacitor is old, or near end of life.

FIG. 12 illustrates an overlay of the exemplary voltage ripple waveformsshown in FIGS. 8-11.

FIG. 13 illustrates an exemplary data processing flow implemented by theDC monitoring system.

FIG. 14 illustrates the exemplary data processing flow of FIG. 13 wherethe first digital filtering process includes determining the averagevalue of the rectified low voltage three-phase DC voltage waveform overa defined period of time and the second digital filtering processincludes determining the RMS value of the voltage ripple waveform.

FIG. 15 illustrates an example output of the first digital filteringprocess of FIG. 14.

FIG. 16 illustrates an example output of the second digital filteringprocess of FIG. 14.

FIG. 17 illustrates the resulting waveforms from the example test abovewhere the capacitance is 16400 uF.

FIG. 18 illustrates the resulting waveforms from the example test abovewhere the capacitance is 2300 uF.

FIG. 19 illustrates a graph of test results comparing capacitance versusvoltage ripple ratio.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Embodiments of the present application are directed to a DC monitoringsystem. Those of ordinary skill in the art will realize that thefollowing detailed description of the DC monitoring system isillustrative only and is not intended to be in any way limiting. Otherembodiments of the DC monitoring system will readily suggest themselvesto such skilled persons having the benefit of this disclosure.

Reference will now be made in detail to implementations of the DCmonitoring system as illustrated in the accompanying drawings. The samereference indicators will be used throughout the drawings and thefollowing detailed description to refer to the same or like parts. Inthe interest of clarity, not all of the routine features of theimplementations described herein are shown and described. It will, ofcourse, be appreciated that in the development of any such actualimplementation, numerous implementation-specific decisions must be madein order to achieve the developer's specific goals, such as compliancewith application and business related constraints, and that thesespecific goals will vary from one implementation to another and from onedeveloper to another. Moreover, it will be appreciated that such adevelopment effort might be complex and time-consuming, but wouldnevertheless be a routine undertaking of engineering for those ofordinary skill in the art having the benefit of this disclosure.

Embodiments are directed to a DC monitoring system connected to a VFD.The DC monitoring system is configured to measure and analyze VFDoperational characteristics. In some embodiments, the VFD is configuredto receive three-phase input power from a standardized source and toprovide variable frequency three-phase output power to a three-phasemotor. It is understood that the concepts, structures, and functionsdescribed herein related to the DC monitoring system can be applied to asingle-phase VFD. In some embodiments, the VFD and motor operate at amedium voltage. As used herein, low voltage refers to voltages less thanor equal to 2000 VAC, and medium voltage has a voltage level in therange of 2001 VAC to 35,000 VAC, as defined by the NFPA. Operating atmedium voltage requires significant safety precautions, particularlywhen considering the connection and use of the DC monitoring system inthe medium voltage environment.

FIG. 3 illustrates a schematic block diagram of a VFD operating at lowvoltage according to some embodiments. The VFD shown in FIG. 3 issimilar in function to the VFD shown and described in FIG. 2, andincludes a converter (rectifier), a DC bus, an inverter, and a controlmodule that includes control logic. FIG. 3 shows additional features ofa common VFD including an AC choke (optional), a power supply, currentand voltage sensors, and a measurement module. The current sensors andthe voltage sensors sense the current and voltage levels at the threeoutputs of the inverter. The measurement block includes circuitry thatreceives and processes the sensed current and voltage signals, which arethen transmitted to the control module. In some embodiments the controlmodule controls operation of the rectifier and the inverter according toprogrammed logic, input control instructions, and sensed data receivedfrom the measurement module. The control module can include circuitry,such as processing circuitry and memory, necessary for all internallogic and decision making requirements for controlling the switchingdevices in the inverter circuit so as to output the three-phase ACvoltage having the specific voltage and frequency for operating thethree-phase motor at an intended speed or torque. Gate drivers used toprovide drive signals to the transistors in the inverter are controlledby the control module. Although the gate drivers are show as separatecircuitry from the control module, the gate drivers can be included aspart of the control module. External connections to the DC bus can beprovided via a link inductor (DC Choke) and brake chopper at terminalsconnection points on the low voltage VFD. In medium voltage VFD's theconnections are not available externally but directly inside the mediumvoltage compartment (bus bars). The DC monitoring system includes ameasurement module, which is connected to the DC bus via the terminalconnection points. The measurement module includes circuitry formeasuring the voltage across the DC bus. This connection to the DC busalso provides power to the measurement module, and as such, in someembodiments, the measurement module may not include an internal powersupply. In this exemplary low voltage application, a low voltagethree-phase AC voltage is supplied at L1, L2, L3. The rectifier convertsthe low voltage three-phase AC voltage to a low voltage DC voltage,which is measured across the DC bus by the measurement module. Theinverter converts the low voltage DC voltage to the specific low voltageAC voltage and frequency to be supplied to the motor, which is outputfrom the VFD at U, V, W as low voltage three-phase AC voltage.

In medium voltage applications, the high voltage levels dictate theparsing of the DC bus into multiple DC buses. FIG. 4 illustrates aschematic block diagram of a medium voltage VFD having multiple DC busesaccording to some embodiments. In the exemplary configuration shown inFIG. 4 there are six DC buses, for example DC bus 1, DC bus 2, etc. Thisconfiguration has three power arms, referred to as modules. Each moduleincludes 2 DC buses, one negative voltage DC bus up to −2000 VDC, onepositive voltage DC bus up to 2000 VDC, and a common zero voltage bus.It is understood that the VFD can be configured to have more or lessthan 6 DC buses, for example up to 15 DC buses or more.

A medium voltage three-phase AC voltage is input to the VFD at H1, H2,H3. The input medium voltage three-phase AC voltage is received at theprimary windings of the transformer TX. The transformer TX has threeprimary windings, one for each of the three-phases, and multiplesecondary windings. The transformer TX is configured to step down andisolate the primary side medium voltage to a secondary side low voltage.Each secondary winding is coupled to a corresponding rectifier thatconverts the low voltage three-phase AC voltage output from thesecondary winding to a rectified low voltage three-phase DC voltage. Therectifiers are configured as series connected pairs, each pair connectedto a corresponding DC bus. Two pairs are connected in series to form amodule. Each DC bus includes a capacitor. A measurement module (notshown), such as the measurement module in FIG. 3, is connected to eachDC bus. The capacitor in each DC bus is connected in parallel to acorresponding inverter. The rectified low voltage three-phase DC voltageoutput from the rectifier is applied to the DC bus of the inverter. Therectified low voltage three-phase DC voltage is converted by theinverter to a low voltage three-phase AC voltage. The low voltagethree-phase AC voltages output from each inverter are combined, orstacked, to provide a medium voltage three-phase AC voltage that isoutput to the motor. In this manner, the medium voltage VFD receives amedium voltage input and provides a medium voltage output, although eachDC bus in the multiple bus configuration carries a low voltage. In someembodiments, this makes it possible to use less-expensive mass-producedlow-voltage IGBT's in the VFD. In an exemplary configuration, eachmeasurement module is configured to measure low voltages across each DCbus in the range of approximately 500 VDC to 2000 VDC, with an accuracyof 250 mV and a measurement resolution of 28 mV. Expected operatingtemperature is to be in the range of −20 degree C. to +60 degree C. Byway of an example, Table 1 shows exemplary voltage readings determinedfrom voltage measurements taken across each of the six DC buses of FIG.4. The voltage measurements are processed to determine an averagevoltage during a defined time period, a voltage ripple peak during thetime period, and a voltage ripple ratio. In this example, the voltageripple ratio is the ratio of the voltage ripple peak divided by theaverage voltage. Each of these values, and the method for determiningeach value, is described in greater detail below.

TABLE 1 Voltage Ripple Average Ripple Peak Ratio DC bus 1 1685.96 V11.45 V 0.68% DC bus 2 1692.60 V  9.11 V 0.54% DC bus 3 1694.52 V 11.09V 0.65% DC bus 4 1687.88 V 10.91 V 0.64% DC bus 5 1692.12 V 11.81 V0.70% DC bus 6 1687.08 V 10.82 V 0.64%

In the case of a medium voltage application of the VFD, it is dangerousfor a user to work directly with the medium voltage VFD due to thehigher voltages and power present. Making routing test measurements oneach DC bus section is part of a preventative maintenance plan, but thisis often skipped due to the hazards present. It is desirable for a userto interface with the VFD and the DC monitoring system connected to theVFD at an interface that is within the low voltage compartment. Toaccommodate such a physical separation, the DC monitoring systemincludes a monitoring device, referred to as a PQube monitoring device,that is located in the low voltage compartment with a VFD low voltagecontrol system, and a data transmission network for interconnecting thePQube monitoring device to the measurement modules connected to the VFDin the medium voltage compartment. The PQube monitoring device islocated in the low voltage compartment with the VFD controls. The PQubemonitoring device is powered from a low voltage power source, such as 24VDC/120 VAC from a main line that is readily available in the lowvoltage compartment. FIG. 5 illustrates a functional block diagram ofthe DC monitoring system configured for transmitting data from a mediumvoltage compartment to a low voltage compartment according to someembodiments. The DC monitoring system shown in FIG. 5 is configured fora medium voltage VFD similar to that shown in FIG. 4. The VFD shownincludes six DC busses, labeled as DC bus 1, DC bus 2, etc. A separatemeasurement module, such as the measurement module shown in FIG. 3, isconnected to each DC bus via dedicated terminal connection points foreach DC bus. This connection to the DC bus also provides power to themeasurement module, and as such, in some embodiments, the measurementmodule may not include an internal power supply. Each measurement moduleis also not referenced to ground but is instead floating relative to theDC bus potential. Failure to not float the measurement module referencemay result in the VFD entering a fault mode and not operating correctly.The measurement modules are labeled as measurement module 1, measurementmodule 2, etc. As such, there is a measurement module for each DC bus.One or more fiber optic cables are connected to each measurement module1-6. Each measurement module is configured to sense the voltage levelsat the correspondingly connected DC bus, generate data signalsrepresentative of the sensed voltage levels, and transmit the datasignals over the attached fiber optic cables. Each measurement moduleincludes an appropriate network interface for interconnecting to theconnected fiber optic cable. In the low voltage compartment, each of thefiber optic cables is connected to a fiber optic concentrator or otherappropriate network switch. The PQube monitoring device is connected tothe fiber optic concentrator via a fiber optic interface (FOI). In someembodiments, the PQube monitoring device is connected to the fiber opticinterface by a fiber optic cable. The data signals transmitted over thefiber optic cables are received by the fiber optic concentrator andprovided to the PQube monitoring device. The fiber optic communicationnetwork provides ground isolation and no direct electrical conductionpath from the medium voltage compartment into the low voltagecompartment. The dashed line shown in FIG. 5 represents an actualphysical barrier, such as a wall, that separates the medium voltagecompartment and the low voltage compartment. A hole can be made in thephysical barrier to enable the fiber optic cables connecting themeasurement modules and the fiber optic concentrator to pass through. Noelectrically conductive pathways are to exist connecting the mediumvoltage compartment to the low voltage compartment. Also, in the lowvoltage compartment, there is a power supply and a backup power supply,such as an uninterruptable power supply (UPS), connected to the PQubemonitoring device.

The PQube monitoring device includes computer processing and memorystorage circuitry for converting the received data signals tocorresponding sensed voltage values and for processing and analyzingthese voltage values to determine corresponding rectified three-phasevoltage waveforms on each of the DC buses in the VFD, and fordetermining filtered values (such as averages) of the rectifiedthree-phase voltage waveforms, voltage ripple waveforms, and calculatingand storing the voltage ripple ratio result. The PQube monitoring devicecan also be connected to an external network, such as by an ethernetTCP/IP network or ModBus TCP or other type of network connection, wiredor wireless. The PQube monitoring device can be further configured forexternal control, storage, and power connections.

VFDs suffer from a common failure mode, namely DC bus capacitor failurein the case of voltage sourced VFDs. In medium voltage applications,large capacitors in the DC bus are required. DC bus capacitor failure insuch medium voltage configurations can lead to extremely dangerousconditions. Detecting the on-set of this failure is desirable to preventthe extensive damage that can be caused by a high voltage, high energyexplosion that can result from this type of failure. The DC monitoringsystem enables measurement and analysis of VFD operationalcharacteristics to determine possible failure conditions and initiateearly failure warning. Such measurement, analysis, and determination canbe applied to all VFD sizes and voltages without complex calculationsand setup requirements.

It some embodiments, the DC monitoring system is part of a largerthree-phase electrical system that includes a VFD connected to athree-phase motor. The VFD converts a three-phase AC voltage receivedfrom the electrical grid to another three-phase AC voltage suitable foroperating a connected three-phase motor. An operational speed of thethree-phase motor is dictated by the voltage and frequency of thethree-phase AC voltage output by the VFD. The speed of the motor can beadjusted by the VFD adjusting the voltage and frequency of thethree-phase AC voltage supplied to the motor. The input three-phase ACvoltage has a standardized frequency, such as 50 or 60 Hz, and theoutput three-phase AC voltage has a specific voltage and frequency tooperate the motor at a specific speed.

In the medium voltage application, the medium voltage three-phase ACvoltage, such as 4160 VAC, is input to the VFD. As described above inregard to FIG. 4, the input medium voltage three-phase AC voltage isreceived at the primary windings of the transformer in the VFD, whichsteps down the primary side medium voltage to a secondary side lowvoltage across multiple different secondary windings, each connected toa corresponding rectifier and DC bus. FIG. 6 illustrates exemplary lowvoltage three-phase AC voltage waveforms across each secondary windingin the medium voltage VFD or input waveforms to a low voltage VFDaccording to some embodiments. The three-phase AC voltage waveformsincludes a phase A voltage waveform, a phase B voltage waveform that is120 degrees out of phase with the phase A voltage waveform, and a phaseC voltage waveform that is 120 degrees out of phase with the phase Bvoltage waveform. Each of the exemplary three-phase AC voltage waveformsshown in FIG. 6 have a voltage level of 480 VAC and frequency of 60 Hz.It is understood that alternative three-phase AC voltages can be usedhaving different voltages and frequencies than the example of FIG. 6.

The low voltage three-phase AC voltage is supplied as input to theconverter of the VFD. The converter rectifies the input low voltagethree-phase AC voltage and outputs a corresponding rectified low voltagethree-phase DC voltage. FIG. 7 illustrates a rectified low voltagethree-phase DC voltage waveform corresponding to the low voltagethree-phase AC voltage waveforms of FIG. 6. The rectified low voltagethree-phase DC voltage waveform includes two components: a DC componentand a voltage ripple component, simply referred to as voltage ripple.Voltage ripple is commonly referred to as an AC component.

The DC component is also known as the zero harmonic DC voltage value orDC offset, which can simply be referred to as the DC voltage value. Therectified low voltage three-phase DC voltage waveform fluctuates aboutthe fixed DC voltage value (DC component). This fluctuation is thevoltage ripple, which is an artifact of the AC-to-DC conversionperformed by the rectifiers. Voltage ripple generates unwantedharmonics, which can be reduced by increasing the capacitance of thecapacitor in the DC bus. The capacitance value is a function of currentdemand (motor torque). As the capacitance decreases with age the currentdraw (seen as ripple) increases. This overloads the capacitor and itoverheats, which can result in capacitor failure. By way of comparison,three different voltage ripple waveforms are shown in FIGS. 8-11. FIG. 8illustrates a voltage ripple waveform corresponding to the rectified lowvoltage three-phase DC voltage waveform of FIG. 7 without a capacitor inthe DC bus. The voltage ripple waveform is obtained by subtracting out,or removing, the fixed DC voltage value (DC component).

The voltage ripple waveform shown in FIG. 8 illustrates the peak to peakvoltage ripple, which represents a worst-case scenario and alsorepresents the case of the DC bus having zero capacitance. FIG. 9illustrates an exemplary voltage ripple waveform where the DC bus has anew capacitor. However, as capacitors age, their capacitance decreases,resulting in an increase in voltage ripple. FIG. 10 illustrates anexemplary voltage waveform where the DC bus capacitor is aged. “Aged” isconsidered an intermediate time between new and end of life. The voltageripple waveform of FIG. 10 corresponds to the same DC bus capacitorapplied as in the voltage ripple waveform of FIG. 9, except thecapacitor is aged compared to being new. FIG. 11 illustrates anexemplary voltage ripple waveform where the DC bus capacitor is old, ornear end of life. The voltage ripple waveform of FIG. 11 corresponds tothe same DC bus capacitor applied as in the voltage ripple waveform ofFIG. 9, except the capacitor is old compared to being new. VFDmanufactures typically suggest replacing capacitors every 5-7 yearsregardless of their performance, which is expensive and requiressignificant downtime of the VFD. FIG. 12 illustrates an overlay of theexemplary voltage ripple waveforms shown in FIGS. 8-11. As can be seenfrom the comparison of the voltage ripple waveforms, the voltage ripplecan be significantly reduced by capacitor filtering, as shown bycomparing the unfiltered voltage ripple waveform to the voltage ripplewaveform filtered with new capacitors, but the filter effectivenessdiminishes over time, as shown by comparing the voltage ripple waveformfiltered with new capacitors to the voltage ripple waveform filteredwith old capacitors. Table 2 shows the corresponding RMS values of thevoltage ripple waveforms shown in FIG. 12.

TABLE 2 Unfiltered (DC Filtered with Filtered with Filtered withremoved) New Capacitors Aged Capacitors Old Capacitors 66.40 V (RMS)1.43 V (RMS) 10.49 V (RMS) 28.89 V (RMS)

Eventually, the diminished capacitance of aging capacitors reaches acritical point, leading to DC bus failure. Capacitors lose capacitancedue to two main reasons: heat, and high current draw that leads to heat.Eventually, the electrolyte in the capacitor evaporates or boils leadingto an internal short and in some cases an explosion. Conventionally, thevoltage level of the DC bus, such as the RMS voltage level, ismonitored. If the monitored voltage level drops below a certain value,then an alarm is sounded indicating a possible DC bus failure. However,this method is supply and load dependent. In other words, if the inputvoltage sags, then the voltage level of the DC bus also sags and a falsealarm may result. The case is similar for load, if the load increasesthe DC bus may reduce and input AC voltage will drop according to thefeeder line impedance. Monitoring of the DC bus voltage level, and moreparticularly, monitoring of the RMS voltage value of the filteredvoltage ripple waveform, also fails to adequately identify decreasedcapacitance levels may result in a DC bus failure threshold. This is dueto the fact that the RMS voltage value of the filtered rectifiedthree-phase DC voltage remains substantially the same even as filtercapacitance decreases and voltage ripple increases. The conventionalmethodology is heavily load dependent and provides a measure of loadconditions, and is not an effective measure of early capacitor failure.

Instead of simply measuring DC voltage values and determining the RMSvoltage value of the filtered rectified three-phase DC voltage waveform,the DC monitoring system also determines the voltage ripple and uses aratio of the voltage ripple as a basis for generating an early capacitorfailure warning. Although the voltage ripple value does provide anindication of potential capacitor failure in the DC bus, the voltageripple value is an absolute value, the ranges and failure indicatingthresholds of which will vary from application to application asoperating voltage requirements and load conditions change. A means fornormalizing the use of voltage ripple measurements across various inputvoltages and applications is to use a voltage ripple ratio as anindicator for potential capacitor failure in the DC bus. The voltageripple ratio provides a dimensionless value. In general, the voltageripple ratio is defined as:

voltage ripple ratio=(voltage ripple value)/(DC voltage value)*100%  (1)

where the voltage ripple value is a determined value of the voltageripple waveform over a defined period of time, and the DC voltage valueis a determined value of the rectified three-phase DC voltage waveformover the defined period of time. The DC monitoring system includesprogram logic and algorithms for performing the described analysis,calculations, and determinations. The rectified three-phase DC voltagewaveform is digitally processed by the DC monitoring system, such as bythe measurement module or the PQube monitoring device, to determine thevoltage ripple value and the DC voltage value. The rectified three-phaseDC voltage waveform can be digitally processed by filtering or otherdigital processing techniques.

FIG. 13 illustrates an exemplary data processing flow implemented by theDC monitoring system. Sensed DC bus data measured by the DC monitoringsystem is digitized, sampled, and filtered to determine the voltageripple value and the DC voltage value, which are then used to calculatethe voltage ripple ratio. The measured voltage levels from the DC busare sensed as analog signals that are converted to digital values by ananalog-to-digital converter (ADC). An exemplary waveform sensed at theDC bus is the rectified low voltage three-phase DC voltage waveformshown in FIG. 7. In some embodiments, the ADC is included in themeasurement module. After the sensed DC bus voltage is sensed andconverted, the digitized signal is transmitted via the fiber optic cableto the fiber optic concentrator in the low voltage compartment. In someembodiments, two separate filtering processes are performed on thedigitized signal. A first digital filtering process isolates the DCvoltage value, referred to as a DC isolation filter. The result of thefirst digital filtering process is a determined DC voltage value, whichis applied in equation (1). A second digital filtering process removesthe determined DC voltage value from the digitized signal, therebyisolating the voltage ripple waveform. An exemplary output of the seconddigital filtering process is the voltage ripple waveform shown in eitherof FIGS. 8-11. A voltage ripple value is determined from the isolatedvoltage ripple waveform. The determined voltage ripple value is appliedin equation (1). Dividing the determined voltage ripple value by thedetermined DC voltage value results in the voltage ripple ratio. Ascaling factor can be used, such as multiplying by 100%, to obtain amore manageable value for the voltage ripple ratio.

The data processing flow shown and described regarding FIG. 13 detailsgeneralized digital filtering processes. The first digital filteringprocess can implement a variety of different digital filtering processesfor determining the DC voltage value from the digitized signal. In someembodiments, the first digital filtering process isolates the DC voltagevalue by taking an average value of the rectified low voltagethree-phase DC voltage waveform over a defined period of time, such as aone-half cycle of the rectified low voltage three-phase DC voltagewaveform. For example, the defined period of time can be 8.333 msec in a60 Hz application, or 10.0 msec in a 50 Hz application. It is understoodthat processes other than taking the average value of the rectified lowvoltage three-phase DC voltage waveform can be implemented including,but not limited to, an RMS value of the rectified low voltagethree-phase DC voltage waveform, or an average of the peak values of therectified low voltage three-phase DC voltage waveform. Similarly, thesecond digital filtering process can implement a variety of differentdigital filtering processes for determining the voltage ripple value ofthe digitized signal. In some embodiments, the second digital filteringprocess determines the voltage ripple value by taking an RMS value ofthe voltage ripple waveform over the defined period of time. It isunderstood that processes other than taking the RMS value of the voltageripple waveform can be implemented including, but not limited to, anaverage value of the voltage ripple waveform, or an average of the peakvalues of the voltage ripple waveform. FIG. 14 illustrates the exemplarydata processing flow of FIG. 13 where the first digital filteringprocess includes determining the average value of the of the rectifiedlow voltage three-phase DC voltage waveform over a defined period oftime and the second digital filtering process includes determining theRMS value of the voltage ripple waveform. An example output of the firstdigital filtering process of FIG. 14 is the average DC voltage waveformshown in FIG. 15. The average DC voltage waveform has a constant value,referred to as an average DC voltage value. An example output of thesecond digital filtering process of FIG. 14 is the RMS voltage ripplewaveform shown in FIG. 16, which is determined from the voltage ripplewaveform of FIG. 8. The RMS voltage ripple waveform has a constantvalue, referred to as a RMS voltage ripple value. Equation (1) can bemodified to account for the specific digital filtering processes used inFIG. 14. The modified equation (1) becomes:

voltage ripple ratio=(RMS voltage ripple value)/(average DC voltagevalue)*100%  (2).

The Applicant has found that monitoring of the voltage ripple ratioprovides a forward indicator of capacitor failure. In both low voltageand medium voltage VFDs a comparison of the voltage ripple ratios forall the DC busses can be made at a glance, easily identifying the poorperforming capacitor, explained in greater detail below.

Table 3 shows resultant calculations for the average DC voltage, the RMSvoltage ripple, and the voltage ripple ratio for the waveforms shown inFIGS. 7-11. The column No Cap corresponds to the worst-case conditionwhere the DC bus filter has no capacitance, as shown in the waveform inFIG. 8. The column New Cap corresponds to the condition where thecapacitors in the DC bus filter are new, as shown in the waveform inFIG. 9. The column Aged Cap corresponds to the condition where thecapacitors in the DC bus filter are at an intermediate stage of use, asshown in the waveform of FIG. 10. The column Old Cap corresponds to thecondition where the capacitors in the DC bus filter are old (near end oflife), as shown in the waveform in FIG. 11. The values for the voltageripple ratio are calculated using equation (2).

TABLE 3 No Cap New Cap Aged Cap Old Cap average DC 648.81 648.81 648.81648.81 voltage (V) RMS voltage  66.40  1.43  10.49  28.89 ripple (V)voltage ripple ratio 10.23% 0.22% 1.62% 4.45%

The DC monitoring system can be used to signal the VFD controller toturn off or restrict the load if a high voltage ripple ratio isdetermined, and to limit the load of the VFD if an excessive voltageripple ratio is determined. The DC monitoring system can also be used tocombine these aspects. Firstly to limit the load, and secondly if a highvoltage ripple ratio is determined at the lower load then the VFD isturned off.

The control module can be configured with a failure warning thresholdfor the voltage ripple ratio. In some embodiments, a voltage rippleratio of 2.0% is set as the failure warning threshold. If at any giventime the calculated voltage ripple ratio is 2.0% or higher, a warningsignal is triggered by the control module. The triggered warning signalcan initiate a local alarm, such as an alarm sounded by the PQubemonitoring device or another device locally connected to the PQubemonitoring device, or a remote alarm, such as a device remotelyconnected to the PQube monitoring device by a communications network.The PQube monitoring device can also take a ‘snapshot’ of all thewaveforms during the event and store them locally, and if connected to acomputer network with access to email, the PQube monitoring device willemail, or otherwise communicate, the digital snapshot to the requiredpersonnel. The Applicant has found that a voltage ripple ratio of 2.0%or higher provides an effective predictor for near-term DC bus capacitorfailure, and a failure warning threshold set at a voltage ripple ratioof 2.0′% provides an effective early warning system. It is understoodthat the failure warning threshold can be set at a different voltageripple ratio than 2.0%. Although use of the voltage ripple ratio isintended to normalize early warning failure triggers characterized bythe voltage ripple measurements across different applications, thefailure warning threshold can be set at different levels for differentapplications, as generalized or customized as desired. For example, acertain type of motor and/or VFD, or motor and VFD combination, may haveone failure warning threshold value, while another type of motor and/orVFD, or another motor and VFD combination, may have a different failurewarning threshold. In some embodiments, in addition to or in place oftriggering an alarm, the warning signal can be used to automaticallyturn off or place in standby the VFD and/or the motor.

Another predictor of possible DC bus capacitor failure can be applied toVFDs having multiple DC buses by comparing determined voltage ripplecharacteristics between various ones or all of the DC buses. In someembodiments, it is the voltage ripple ratio for each DC bus that iscompared from one DC bus to another. In other embodiments, it is thevoltage ripple value, such as the RMS voltage ripple value, for each DCbus that is compared from one DC bus to another. A VFD with multiple DCbuses is configured for balanced load, which translates to equal voltageripple ratios or voltage ripple values across each DC bus. If thedetermined voltage ripple characteristic, such as the voltage rippleratio or the voltage ripple value, is different across one or more DCbuses compared to the other DC buses, then an unbalanced conditionexists. The unbalanced condition can be a predictor of a potential DCbus capacitor failure and provides justification for generating awarning signal. Control logic, either in the PQube monitoring device oran electronic device remotely connected to the PQube monitoring device,is configured with programmed logic and memory for making suchcomparisons and generating appropriate warning signals. By way ofexample, consider the VFD of FIG. 4 that has six DC buses. Consider thatthe voltage ripple characteristic to be measured and compared is thevoltage ripple ratio for each DC bus. The voltage ripple ratio for DCbus 1 is referred to as A, the voltage ripple ratio DC bus 2 is referredto as A−, the voltage ripple ratio for DC bus 3 is referred to as B, thevoltage ripple ratio for DC bus 4 is referred to as B−, the voltageripple ratio for DC bus 5 is referred to as C, and the voltage rippleratio for DC bus 6 is referred to as C−. The comparison logic can be asfollows:

Compare A to A−

-   -   Expected result of A+(A−)=0    -   Else unbalanced and early warning signal generated.

Similarly, compare B to B− and C to C−.

Different comparison logic can be used as an alternative or addition tothe above comparison logic. For example:

-   -   Compare all six values A, A−, B, B−, C, C− to each other        -   Expected result A+(A−)+B+(B−)+C+(C−)=0        -   Else further diagnostics are required.

Additional diagnostics may include:

Compare A to B

-   -   Expected result A−B=0    -   Else unbalanced and early warning signal generated.

Similarly compare A to C and B to C.

Similarly compare A−, B− and C−

Unbalance across DC buses may occur due to an unbalance in the inputthree-phase AC voltage, this must be considered as part of theadditional diagnostics.

Example

The following example shows actual results performed in a low voltageenvironment for a single DC bus configuration. This test was in a lowvoltage environment for safety reasons as it is too dangerous to performthis test in a medium voltage environment. The tests results provide auseful basis for evaluation a multiple bus configuration in a mediumvoltage environment since individual buses in a multiple busconfiguration have similar data points as the single bus test resultsshown below.

A 200 Hp VFD is selected to represent a common drive size. The VFD isconnected to a 480V, 60 Hz input power source on R, S, and T terminalsof the VFD. The outputs (U, V, and W) of the VFD are connected to a 200Hp rotating load. The VFD has eight 8200 uF capacitors connected ingroups of two serially connected, and in a bank of 4 parallel pairs.This results in a total capacitance of 16400 uF.

The following test steps are followed, and the data recorded. After eachstep the drive is powered down and the capacitor bank allowed todischarge before making modifications to the DC bus (removing ofcapacitance). All electrical data including the DC bus status isrecorded in a Power Standards Lab PQube 3 (PQube monitoring device) witha 1000V Attenuator module for the DC measurements. The test stepsinclude:

-   -   1) Start the VFD and load up to full load (240 A to the motor).    -   2) Capture the following data:        -   a. All AC (voltage ripple) parameters, V, A, Hz, PF        -   b. Harmonics        -   c. DC bus average and peak ripple    -   3) Power down the VFD    -   4) Remove approximately 25% of the capacitance from the DC bus    -   5) Power up the VFD    -   6) Return to full load    -   7) Capture data    -   8) Repeat        Note: The test was stopped by the VFD technician, who indicated        the remaining capacitors where hot and likely to explode if the        test continued. This occurred with about 18% capacitance        remaining. Although the preceding procedure is shown in the        context of a low voltage environment, the same procedure can be        applied in a medium voltage environment having multiple buses.

The following results correspond to the load being driven at full load.FIG. 17 illustrates the resulting waveforms from the example test abovewhere the capacitance is 16400 uF. A capacitance value of 16400 uFcorresponds to a “new capacitor” condition. The first (top) waveformshows the low voltage three-phase AC voltage waveform across a secondarywinding of the transformer. The second waveform shows the current inputinto the VFD. The third waveform shows the voltage ripple waveform asmeasured at the DC bus. The fourth (bottom) waveform shows the rectifiedlow voltage three-phase DC voltage waveform, the same as shown in FIG.7.

FIG. 18 illustrates the resulting waveforms from the example test abovewhere the capacitance is 2300 uF. A capacitance value of 2300 uFcorresponds to an “old capacitor” condition.

Table 4 below shows resultant calculations for the average DC voltage,the RMS voltage ripple, and the voltage ripple ratio for variouscapacitance values tested.

TABLE 4 Capacitance (uF) 16400 12300 8200 4100 2300 RMS voltage ripple(V)  6.25  6.82  7.07  10.15  12.73 average DC voltage (V) 623.03 621.90622.47 622.20 621.27 voltage ripple ratio 1.02% 1.11% 1.14% 1.65% 2.12%

FIG. 19 illustrates a graph of test results comparing capacitance versusvoltage ripple ratio. The solid line represents the voltage ripple ratiodetermined using the digital filtering processes described above. Thedashed line represents the voltage ripple ratio calculated using anappropriate polynomial equation. Table 4 and the graph of FIG. 19indicate how the voltage ripple ratio increases with loss ofcapacitance. More particularly, the voltage ripple ratio remainsslightly above 1% until the capacitance is reduced to 8000 uF. Afterthis point the voltage ripple ratio increases rapidly as more capacitorsare removed, which corresponds to reduced capacitance value. By the timethe voltage ripple ratio reached 2%, the technician indicated theremaining capacitors were getting hot, and on his recommendation thetest was concluded.

The present application has been described in terms of specificembodiments incorporating details to facilitate the understanding of theprinciples of construction and operation of the DC monitoring system.Many of the components shown and described in the various figures can beinterchanged to achieve the results necessary, and this descriptionshould be read to encompass such interchange as well. As such,references herein to specific embodiments and details thereof are notintended to limit the scope of the claims appended hereto. It will beapparent to those skilled in the art that modifications can be made tothe embodiments chosen for illustration without departing from thespirit and scope of the application.

What is claimed is:
 1. A system to measure operational characteristicsof a variable frequency drive, the system comprising: a. a mediumvoltage compartment comprising: i. a medium voltage variable frequencydrive configured to receive as input a medium voltage, wherein themedium voltage variable frequency drive comprises multiple DC buses,each DC bus having a low voltage; ii. a plurality of measurementmodules, one measurement module coupled to one DC bus, wherein eachmeasurement module is configured to measure a voltage value of the DCbus and convert the measured voltage value to a data signal; and iii. aplurality of optical fibers, one optical fiber coupled to onemeasurement module, and each optical fiber configured to transmit thedata signal; b. a low voltage compartment comprising a PQube measurementdevice coupled to the plurality of optical fibers to receive the datasignals, wherein the PQube measurement device is configured to processthe received data signals to determine operational characteristics ofeach DC bus; and c. a physical barrier separating the medium voltagecompartment and the low voltage compartment, wherein the physicalbarrier is configured to allow the plurality of optical fibers to passtherethrough.
 2. The system of claim 1 wherein the medium voltagecomprises a voltage value in the range of 2001 VAC to 35,000 VAC.
 3. Thesystem of claim 1 wherein the low voltage comprises a voltage value inthe range of 0 to 2000 VAC.
 4. The system of claim 1 wherein each DC buscomprises a filtering capacitor, and the measured voltage value of theDC bus corresponds to a voltage value across the filtering capacitor. 5.The system of claim 1 wherein the measurement module is powered by theDC bus.
 6. The system of claim 1 wherein the low voltage compartmentfurther comprises a data concentrator module coupled between theplurality of optical fibers and the PQube measurement device.
 7. Asystem to measure operational characteristics of a variable frequencydrive, the system comprising: a. a medium voltage compartmentcomprising: i. a medium voltage variable frequency drive configured toreceive as input a medium voltage in the range of 2001 VAC to 35,000VAC, wherein the medium voltage variable frequency drive comprisesmultiple DC buses, each DC bus having a low voltage in the range of 0VDC to 2000 VDC; ii. a plurality of measurement modules, one measurementmodule coupled to one DC bus, wherein each measurement module isconfigured to measure a voltage value of the DC bus and convert themeasured voltage value to a data signal, further wherein eachmeasurement module is powered by the one DC bus; and iii. a plurality ofoptical fibers, one optical fiber coupled to one measurement module, andeach optical fiber configured to transmit the data signal; b. a lowvoltage compartment comprising a PQube measurement device coupled to theplurality of optical fibers to receive the data signals, wherein thePQube measurement device is configured to process the received datasignals to determine operational characteristics of each DC bus; and c.a physical barrier separating the medium voltage compartment and the lowvoltage compartment, wherein the physical barrier is configured to allowthe plurality of optical fibers to pass therethrough to transmit thedata signals from a medium voltage compartment to the low voltagecompartment.
 8. The system of claim 7 wherein each DC bus comprises afiltering capacitor, and the measured voltage value of each DC buscorresponds to a voltage value across the filtering capacitor.
 9. Thesystem of claim 7 wherein each measurement module is powered by thecorresponding one DC bus.
 10. The system of claim 7 wherein the lowvoltage compartment further comprises a data concentrator module coupledbetween the plurality of optical fibers and the PQube measurementdevice.